Touch-sensitive components such as touch screens are an increasingly common feature of opto electronic displays e.g. in items such as computers, mobile phones, etc. Such displays require an electrode on the front surface of the display, which must be substantially transparent, in order not to block light transmission from the display and so to enable the display to perform its intended function. A common approach is to use a transparent conductor such as Indium Tin Oxide (ITO) as this electrode. ITO suffers from a number of drawbacks. It is brittle. It is relatively expensive, due to the scarcity of indium. It has high resistivity (typical layers have a sheet resistivity in the range 100-1000 Ohms per square). The high resistivity means that extra processing steps are required to make connections to the grid. It is difficult to etch, making further processing expensive.
An alternative approach is to use an array or grid of fine metal lines, wires or tracks, forming an electrically conductive mesh micropattern disposed on the front surface of the display as the front electrode. The metal grid may be made with very thin wires, e.g. less than 10 micron in width, using standard photolithographic processing. Provided sufficiently large gaps are left between the fine wires of the grid, the wires block very little of the light from the display, and the wires not readily visible to the eye.
A metal grid is malleable, allowing the use of flexible substrates. Thin copper layers typically have a sheet resistivity in the range of 100 mOhms per square. The low resistivity means that bulk metal connections to the grid can be made using the same layer as the fine mesh, reducing the number of processing steps (and hence cost).
A drawback of using metal wires is that the reflective nature of the metal can make the mesh highly visible to a user, which is undesirable.
WO 2009/108758 of 3M concerns touch screen sensors with such conductive micropatterns, and discloses a number of methods of reducing the visibility of a metallic mesh.
The present invention concerns an alternative approach to reducing the visibility of a copper mesh.
There is a good deal of prior art on blackening treatments for copper and copper alloys. Historically brass optical devices such as telescopes were blackened to reduce internal reflections either by the use of matt black paint, or by oxidising processes to produce thick opaque layers. Recently the manufacture of plasma display panels (PDPs) has required the use of fine metal grids to reduce electromagnetic emissions. These are blackened to reduce visibility to the user.
U.S. Pat. No. 2,460,896 discloses a method of blackening copper-based optical components using a mixture of sodium chlorite and sodium hydroxide. The specification notes that “Heretofore, copper and copper alloy surfaces have been blackened by the so-called ‘oxidising process’ in which the surface is cleaned and a copper sulfide film is formed by immersing the surface in solutions of sodium sulfide or ammonium sulfide or other water soluble sulfides. This produced brown or black coatings on the surface.”.
JP2005139546 discloses the use of an electrolytic solution of cobalt sulfate to form a resilient black surface layer on copper foil and copper mesh suitable for a PDP.
Other recent examples, e.g. JP 2010010179, JP2009231426, use the electrolytic deposition of a black nickel-tin oxide to achieve the same result.
JP2009218368 uses electrolytic deposition of copper oxide from a solution of sodium hydroxide or potassium hydroxide to deposit a layer of copper oxide of 0.6 to 3 micron in thickness. This surface treatment is applied to printed circuit boards in particular.
EP 0963416 discloses a transparent member for use as a shield against electromagnetic waves with a copper layer having a brown to black coloured surface layer e.g. of copper sulfide. The thickness of the surface layer is not disclosed and the issue of regulation and control of the thickness is not addressed.
GB 1012224 discloses a transparent electronically conductive film in which a cadmium sulfide layer 250 Angstroms thick is converted to copper sulfide.
GB 986697 discloses preparation of transparent electrically conducting copper sulfide films by contacting a copper film with a sulfur-containing gas.